This invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel, to maintain the patency thereof. These devices are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy, laser angioplasty or other means.
Several interventional treatment modalities are presently used for heart disease, including balloon and laser angioplasty, atherectomy, and by-pass surgery. In typical coronary balloon angioplasty procedures, a guiding catheter having a distal tip is percutaneously introduced through the femoral artery into the cardiovascular system of a patient using a conventional Seldinger technique and advanced within the cardiovascular system until the distal tip of the guiding catheter is seated in the ostium of a coronary artery. A guide wire is positioned within an inner lumen of a dilatation catheter and then both are advanced through the guiding catheter to the distal end thereof. The guide wire is first advanced out of the distal end of the guiding catheter into the patient""s coronary vasculature until the distal end of the guidewire crosses a lesion to be dilated, then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient""s coronary anatomy over the previously introduced guide wire until the balloon of the dilatation catheter is properly positioned across the lesion. Once in position across the lesion, the balloon is inflated to compress the plaque of the lesion against the inside of the artery wall and to otherwise expand the inner lumen of the artery. The balloon is then deflated so that blood flow can be resumed through the dilated artery and the dilatation catheter can be removed therefrom. Further details of dilatation catheters, guide wires, and devices associated therewith for angioplasty procedures can be found in U.S. Pat. No. 4,323,071 (Simpson-Robert); U.S. Pat. No. 4,439,185 (Lindquist); U.S. Pat. No. 4,516,972 (Samson); U.S. Pat. No. 4,538,622 (Samson, et al.); U.S. Pat. No. 4,554,929 (Samson, et al.); U.S. Pat. No. 4,616,652 (Simpson); U.S. Pat. No. 4,638,805 (Powell); U.S. Pat. No. 4,748,982 (Horzewski, et al.); U.S. Pat. No. 5,507,768 (Lau, et al.); U.S. Pat. No. 5,451,233 (Yock); and U.S. Pat. No. 5,458,651 (Klemm, et al.).
One problem that can occur during balloon angioplasty procedures is the formation of intimal flaps which can collapse and occlude the artery when the balloon is deflated at the end of the angioplasty procedure. Another problem characteristic of balloon angioplasty procedures is the large number of patients who are subject to restenosis in the treated artery. In the case of restenosis, the treated artery may again be subjected to balloon angioplasty or to other treatments such as by-pass surgery, if additional balloon angioplasty procedures are not warranted. However, in the event of a partial or total occlusion of a coronary artery by the collapse of a dissected arterial lining after the balloon is deflated, the patient may require immediate medical attention, particularly in the coronary arteries.
A focus of recent development work in the treatment of heart disease has been directed to endoprosthetic devices called stents. Stents are generally cylindrically shaped intravascular devices which are placed within an artery to hold it open. The device can be used to reduce the likelihood of restenosis and to maintain the patency of a blood vessel immediately after intravascular treatments. In some circumstances, they can also be used as the primary treatment device where they are expanded to dilate a stenosis and then left in place. Further details of stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); U.S. Pat. No. 4,886,062 (Wiktor); U.S. Pat. No. 5,421,955 (Lau); and U.S. Pat. No. 5,569,295 (Lam).
A variety of stent designs have been developed and include coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from expandable heat sensitive metals; self-expanding stents inserted in a compressed state for deployment in a body lumen, and stents shaped in zig zag patterns. One of the difficulties encountered using prior art stents involve maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery and accommodate the often tortuous path of the patient""s vasculature. Generally, the greater the longitudinal flexibility of the stent, the easier and more safely it can be delivered to the implantation site.
Various means have been described to deliver and implant stents. One method frequently described for delivering of a stent to a desired intraluminal location includes mounting the stent on an expandable member, such as a balloon on a distal end of an intravascular catheter, advancing the catheter to the desired location within the patient""s vascular system, inflating the balloon on the catheter to expand the stent into a permanent expanded condition. The expandable member is then deflated and the catheter is withdrawn, leaving the expanded stent within the blood vessel, holding open the passageway thereof. Other prior art stent delivery catheters used for implanting self-expanding stents include an inner member upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is placed over the compressed stent to maintain it in its compressed state prior to deployment. When the stent is to be deployed in the body vessel, the outer restraining sheath is retracted in relation to the inner lumen to uncover the compressed stent, allowing the stent to move into its expanded condition for implantation in the patient.
Advancing the stent through a patient""s vasculature, which can involve traversing sharp bends and other obstacles, may require the stent to be highly flexible. While it is beneficial if the area of treatment is located in a substantially straight portion of the patient""s vasculature, often, the area of treatment is at a curved portion of the body vessel which can be problematic to the physician when implanting the device. Therefore, stent flexibility must permit the stent to be deployed in and conform to a tortuous section of a patient""s vasculature. Moreover, once implanted, the stent should not attempt to straighten the curved vessel since this could possibly cause disruption in the normal flow of the blood through the vessel and could possibly result in restenosis occurring at that location. Additionally, visualization of the stent with a fluoroscope, which is currently the most widely used method of locating and positioning the stent during deployment, requires a stent with good radiopacity.
Different methods have been attempted to give stents high flexibility and radiopacity. By making stents out of relatively thin material, flexibility can be increased. However, the use of thin material can reduce the radiopacity of the stent, making it more difficult for the physician to visualize the stent. Conversely, the use of thicker material, which can promote radiopacity results in reduced stent flexibility, which can impair the deliverability of the stent. When the stent is made from a self-expanding material, such as nickel titanium, which has less radiopacity than a stainless steel stent, for example, the problem of visualizing the self-expanding stent can be further increased if thinner material is used to increase flexibility.
An early attempt at achieving a flexible stent with good radiopacity characteristics involved providing a stent of a base material with good flexibility and strength but relatively low radiopacity, and then adding a thin layer of a highly-radiopaque material, such as gold, to the stent. This approach, which required the use of two separate materials, involved a relatively complicated process in applying the radiopaque material to the stent. Additionally, the use of multiple materials can complicate use and deployment of the stent, particularly where the different materials have different material characteristics, such as different strengths, different biocompatibility, or different responses to temperature changes.
Another approach was to provide a stent with substantially thicker portions at each end. Such an approach provided a stent with highly radiopaque ends, so that a physician could easily view the stent ends during stent delivery.
What has been needed and heretofore unavailable is an improved means of providing a stent with high flexibility, strength, and radiopacity. The present invention satisfies these and other needs.
The present invention provides a stent with increased bendability and flexibility without comprising the radial strength of the device. The stents of the present invention have sufficient longitudinal flexibility along their longitudinal axis to facilitate delivery through tortuous body lumens, yet remain stable when expanded radially to maintain the patency of a body lumen such as an artery or other vessel, when implanted therein. The present invention in particular relates to unique patterns which permit greater longitudinal flexibility and sufficient radial-expansibility and strength to hold open body lumens. The present invention achieves this enhanced bending through the use of preferential bending points disposed on the stent structure to increase flexibility and bendability when being delivered through the patient""s vasculature and later for implantation in the body vessel, particularly in a curved body vessel.
The stents of the present invention can generally include a plurality of adjacent cylindrical elements (often referred to as xe2x80x9cringsxe2x80x9d) which are generally expandable in the radial direction and arranged in alignment along a longitudinal stent axis. The cylindrical elements can be formed in a variety of serpentine wave patterns transverse to the longitudinal axis and contain a plurality of alternating peaks and valleys. At least one interconnecting member extends between adjacent cylindrical elements and connects them to one another. These interconnecting members insure a minimal longitudinal contraction during radial expansion of the stent in the body vessel. The serpentine patterns may have varying degrees of curvature in the regions of peaks and valleys and are adapted so that radial expansion of the cylindrical elements are generally uniform around their circumferences during expansion of the stent from the contracted condition to an implanted condition.
Again, the present invention achieves increased bendability and flexibility of the stent through the use of preferential bending points created on these interconnecting members which connect adjacent cylindrical rings. In this regard, the bending points help the interconnecting members to bend or flex when needed without compromising the radial strength developed by the cylindrical elements. These bending points on the interconnecting members can be created in a number of different ways. For example, the bending point can be achieved by decreasing the width of the strut in order to have a smaller cross-section at one or more points along the length of the interconnecting member. Additionally, the bending point can be created by decreasing the strut thickness at preferential points along the length of the interconnecting member. The present invention can also utilize a combination of decreased strut widths and strut thicknesses to create a highly flexible bending point along the interconnecting members, as well.
In one aspect of the present invention, the preferential bending points can be located at the ends of the interconnecting members which are directly adjacent to the cylindrical elements. Another preferential bending point can be created at about midpoint of the interconnecting member to enable the distance between the cylindrical elements to be reduced as needed, especially when the stent is being implanted in a body vessel having a tight bend radius. As a result, the bending points allow select interconnecting members to bend and shorten as needed in order to conform to the inside radius of a tight curved body vessel. Other interconnecting members of the stent may remain relatively unbent to provide structural stability to the stent.
The number and location of the interconnecting members and the bending points can be varied in order to develop the desired longitudinal flexibility in the stent structure both in the unexpanded, and expanded positions. The use of fewer interconnecting members usually results in a more flexible design since this xe2x80x9cfrees upxe2x80x9d more of the highly flexible peaks of the cylindrical element. Thus, while stent flexibility is derived mainly from the cylindrical rings, the number and location of the interconnecting members can influence the flexibility by constraining or xe2x80x9cfreeing upxe2x80x9d the peaks and valleys. Generally, the greater the longitudinal flexibility of the stent, the easier and the more safely it can be delivered to the implantation site, especially when the implantation site is on a curved section of a body lumen, such as a coronary artery or peripheral blood vessel, and especially in saphenous veins and larger vessels. However, if increased vessel scaffolding is desired, the number of interconnecting members can be increased as needed.
The resulting stent structures are a series of radially expandable cylindrical elements that are spaced longitunally close enough so that small dissections in the wall of a body lumen may be pressed back into position against the lumenal wall, but not so close as to compromise the longitudinal flexibility of the stent both when negotiating through the body lumens in their unexpanded state and when expanded into position. Each of the individual cylindrical elements may rotate slightly relative to their adjacent cylindrical elements without significant deformation, cumulatively providing stents which are flexible along their length and about their longitudinal axis, but which still are very stable in their radial direction in order to resist collapse after expansion.
The stents of the present invention can be readily delivered to the desired target location by mounting it on an expandable member, such as a balloon, of a delivery catheter and passing the catheter-stent assembly lumen to the target area. A variety of means for securing a stent to the extendible member of the catheter for delivery to the desired location are available. It is presently preferred to crimp or compress the stent onto the unexpanded balloon. Other means to secure the stent to the balloon included providing ridges or collars on the inflatable member to restrain lateral movement, using bioabsorbable temporary adhesives, or adding a retractable sheath to cover the stent during delivery through a body lumen. When a stent of the present invention is made from a self-expanding material, such as nickel titanium alloy, a suitable stent delivery assembly which includes a retractable sheath, or other means to hold the stent in its expanded condition prior to deployment, can be utilized.
The serpentine pattern of the individual cylindrical elements can optionally be in phase with each other in order to reduce the contraction of the stent along its length when expanded. The cylindrical elements of the stent are plastically deformed when expanded (except with NiTi alloys or other self-expanding materials) so that the stent will remain the expanded condition and therefore must be sufficiently rigid when expanded when expanded to prevent the collapsed thereof during use. When the stent is formed from superelastic nickel titanium alloys or similar materials, the expansion occurs when the stress of compression is removed. Shape memory alloys, which also include NiTi, can be used as well. Shape memory alloys allow for phase transformation to occur, resulting in the expansion of the stent.
The above and other objects and advantages of this invention will be apparent from the following more detailed description when taken in conjunction with the accompanying drawings of exemplary embodiments.